Self-igniting fuel and a method of using same



United States Patent SELF-IGNI'IING FUEL AND A METHOD OF I USING SAlVlE Paul Condit and -Manuel A..Pino, Berkeley, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application October 29, 1951,

Serial No. 253,749

7 Claims. (Cl. (ill-35.4)

This invention relates to propellant fuel compositions which are characterized by self-ignition when mixed with powerful oxidizing agents and to particular mixtures of these fuels and oxidizing agents.

Self-igniting compositions of various kinds. have for many years found use in the construction of assorted tools of war for use against personnel and property. More recently, it has been recognized that self-igniting compositions are useful as propellants for rockets, guided missilesjand aircraft powered by jet or thrust engines.

. In the development of fuels for rockets, thrust engines, etc., it has been recognized that the property of selfignition in the propellant fuel, while not absolutely essential; constitutes a major convenience in the construction andope ration of these devices. In the development of rocket fuels, two types have been recognized: monergolic fuels, or monergols,.i. e., unitary compositions which duc'ingcomp'onent and the oxidizing agent be rapid over a wide temperature range; that the reducing component have a high'density so that the volume of fuel tanks require ignition by spark, pyrotechnic charge or otherwise and, once started, they continue to burn on being sprayed into the hot zone; and hypergolic fuels, or hypergols, which comprise two or more components which are separately stored and which'spontaneously ignite upon proper in a combustion chamber. The individual components of the hypergolic mixture are not self-ignit- 1ng. w 7

Monergols so far developed have proved unreliable in that they have all shown a tendency to backfire from the combustion chamber to storage tanks. This. difficulty is not encountered with hypergolic fuels since self-ignition occurs only upon mixing of the individual components which are not themselves self-igniting and neither of which alone has the property of undergoing self-sustained combustionafterjignition.

Hypergolic; propellants may be v generally" described as m ixt uresvof a reducing component and an 'oxidizingcomponent. a short period during which the mixture exists as a composition of matter. During this short periodrelatively slow reactiorr of the two components is apparently initiat'ed and this relatively slow reaction 'isffollowed by spontane'ous'ignition of the fuel. The ignition" delay period is quite short, being substantially less than one second and being preferably not more than about fifty milliseconds. V I; I I v The j'reducingj component of a hypergolic propellant should have certain properties asfollows: It shouldignite spontaneouslyupon being combined with the selected oxidizing agent; the ignition delay period should be short, preferably not exceeding milliseconds; it should have a low freezing point so that it can, be pumpedto the combustion chamber at. the low-temperatures prevailing at high altitudes; it should have a high boiling point 'to facilitate handling and increase its safety in use; it should i When the two components are mixed," there is may be reduced; that the material be non-toxic and easy to handle; that it have a high heat of combustion; and that it be non-corrosive.

It has now been found that the foregoing property requirements are satisfied in a remarkable degreeby a selfigniting propellant composition consisting essentially of at least one material of the group consisting of acyclic mercaptans containing from 1 to 5. carbon atoms, especially primary and secondary mercaptans, and an oxidizing agent consisting predominantly of fuming nitric acid.

. It has further been found that mercaptans produced by separating mercaptans from thermally cracked naphtha and fractionally distilling the separated mercaptans to recover a fraction boiling within the range from 125 F. to 300 F. constitute a highly satisfactory reducing component for a hypergolic propellant. This mercaptan fraction is characterizedin that it is self-igniting when mixe with concentrated nitric acid.

The reducing component of the above-described hyper golic propellant may be methyl mercaptan, ethyl mercaptan, normal propyl mercaptan, isopropyl mercaptan,

normal butyl mercaptan, isobutyl mercaptan, secondary butyl mercaptan, allyl mercaptan, normal amyl mercaptan, secondary amyl mercaptan, unsaturated C3-C5 mercaptans, or a mixture of two or'mo're of these materials. Tertiary butyl mercaptan and tertiary amyl mercaptans are satisfactoryif mixed with at least 50% by volume of by Weight of water is satisfactory. A small amount of N205 may desirably be added to commercial fuming nitric acid to render it anhydrous.

It has been found that the ignition delay period observed when fuming nitric acid is mixed with the mercaptans above 'd escribed may be considerably shortened by incor- I poration in'the nitric acid of'a minor proportion of a material which functions as an ignition accelerator. One such material is nitrogen dioxide which may be added in amounts from 2% to 20%. by .weight, ordinarily about 7 /2% .by Weight beingernployed. The. nitrogen dioxide impartsfacharacteristic-reddish color to the acid and this these products should be aslowas possible order that.

the specific impulse (pounds thrust/pounds propellant/ mixture issometime s referred to as red fuming nitric 'acid; A second material which may be added to fuming nitric acid to accelerate ignition is sulfuric acid. This material is also added in amounts from 2% to 20% by weight, from 12% to 15 by weight being. ordinarily employed.

A third material whichmay be added to fuming nitric acid as-;an ignition'accelerator isnitrosyl sulfuric acid; This material is='also;employed in. amounts from 2% tov 20% by weight, about 15% being desirably employed.

The following examplesillustrate the propellant of'this inventiom- I;

' U yrEXAMPLEnI' iQne volume of ethyl mercaptan and threevolumes of 1 fuming nitric acid containing .15 by weight of sulfuric acid were mixed atf 40 F. Ten milliseconds after mix ing, spontaneous ignition ofthe'mixureoccurred. The mercaptanwasjvery rapidly oxidized forming water vapor, carbon monoxide, carbon dioxidexand sulfur dioxide and thecncid wasreducedforming water vapor, nitrogen and nitrogenoxidesl The ignition delay is measured by mixing Patented June 19,1956

Commercial fuming nitric the mercaptan and the acid in a specially constructed oxidizer cylinder so arranged that at the instant of mixing the mercaptan and the acid, an electrode is grounded initiating a pulse to an electronic timer to commence the measurement of the delay. Soon after the acid'contacts the mercaptan, ignition takes place. Ionization caused by the flame grounds a tubular electrode to the walls of the oxidizer cylinder furnishing the terminating pulse to the electric timer to complete the delay measurement.

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that normal propyl mercaptan was substituted for ethyl mercaptan. The mixture of normal propyl mercaptan and acid spontaneously ignited 16 milliseconds after the mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 3 The procedure of Example 1 was repeated, substituting normal butyl mercaptan for ethyl mercaptan. The mixture of acid and normal butyl mercaptan spontaneously ignited 12 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 4 The procedure of Example 1 was repeated, substituting normal amyl mercaptan for ethyl mercaptan. The mixture of normal amyl mercaptan and acid spontaneously ignited 18 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 5 The procedure of Example 1 was repeated, substituting isobutyl mercaptan for ethyl mercaptan. The mixture of isobutyl mercaptan and acid spontaneously ignited 20 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 6 The procedure of Example 1 was repeated, substituting isopropyl mercaptan for ethyl mercaptan. The mixture of isopropyl mercaptan and acid spontaneously ignited 22 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 7 The procedure of Example 1 was repeated, substituting secondary butyl mercaptan for ethyl mercaptan. The mixture of secondary butyl mercaptan and acid spontaneously ignited 26 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 8 The procedure of Example 1 was repeated, substituting allyl mercaptan for ethyl mercaptan. The mixture of allyl mercaptan and acid spontaneously ignited 14 milliseconds after mixing occurred and burned rapidly forming gaseous products.

EXAMPLE 9 The procedure of Example 1 was repeated, substituting a mixture of mercaptans separated from cracked naphtha for the ethyl mercaptan in the example. The mercaptan mixture and the acid spontaneously ignited 17 milliseconds after mixing occurred.

The mercaptan mixture was prepared in the following manner: The pentane-octane cut of thermally cracked naphtha from a sulfur containing crude oil and boiling from 100 F. to 280 F. was washed with aqueous caustic at a concentration of about 25 B. to remove aliphatic acids and cresylic acids. The naphtha was then washed with a 46 B. solution of caustic in methanol to separate the mercaptans from the hydrocarbons. After contacting the hydrocarbons the methanolic caustic containing extracted mercaptans was stripped with steam to recover an overhead comprising water, methanohmercaptans and oil.

This overhead was settled to separate an upper phase comprising mercaptans and oil and a lower phase comprising aqueous methanol. The upper phase was then fractionally distilled to separate a mixed mercaptan cut boiling from 136 F. to 250 F. The mercaptan fraction contained 1% isopropyl mercaptan, 23% normal propyl mercaptan, 37% secondary butyl mercaptan, 7% of isobutyl mercaptan, 11% normal butyl mercaptan, 21% amyl mercaptans and a trace of tertiary-butyl mercaptan. This fraction is a highly desirable reducing component of a hypergolic fuel, since its content of low molecular weight, high vapor pressure mercaptans is low, its content of non-self-igniting tertiary mercaptans is low and it is essentially free of nonself-igniting mercaptans containing 6 or more carbon atoms.

EXAMPLE 10 The mixed mercaptans described in Example 9 were tested in the manner described in Example 1, at a variety of temperatures, and with several acid compositions. The mercaptan mixture spontaneously ignited upon addition of the several acids and the ignition delays are shown in the following table. In the table the symbols WFNA indicates fuming nitric acid and RFNA indicates fuming nitric containing about 7 /2% by weight of nitrogen dioxide.

Table IGNITION DELAYS OF MIXED BUTYL MERCAPTANS EFFECTS OF TEMPERATURE AND OXIDIZER CYLIN- DRICAL TESTER EXAMPLE 1 l Tertiary-butyl mercaptan was mixed with fuming nitric acid and with fuming nitric acid containing 7 /2% by weight of nitrogen dioxide at room temperature. The mixtures were not self-igniting.

EXAMPLE 12 Tertiary-amyl mercaptan was mixed with fuming nitric acid and with fuming nitric acid containing 7.5% nitrogen dioxide at room temperature. The mixtures were not self-igniting.

EXAMPLE 13 Normal hexyl mercaptan was mixed with fuming nitric acid and with fuming nitric acid containing 7.5% by weight of nitrogen dioxide at room temperature. The mixtures were not self-igniting.

EXAMPLE 14 Normal heptyl mercaptan was mixed with fuming nitric acid and with fuming nitric acid containing 7.5% by weight of nitrogen dioxide at room temperature. The mixtures were not self-igniting.

EXAMPLE 15 Normal decyl mercaptan was mixed with fuming nitric acid and with fuming nitric acid containing 7.5% by weight of nitrogen dioxide at room temperature. The mixtures were not self-igniting.

EXAMPLE 16 The mercaptan mixture described in Example 9 was tested in a thrust engine using fuming nitric acid containing 15% by weight of sulfuric acid as the oxidizing agent. Runs were made at sea level at temperatures of 75 F., l2 F. and --31 F. In all cases the engine started and operated satisfactorily. Runs were made at a pressure altitude of about 50,000 ft. at temperatures from 27 F. to -73 F. Satisfactory starts in performance were obtained in all runs. Eleven successful runs at 50,000 ft. at a temperature of approximately 70 F. clearly demonstrated the reliability of the starting characteristics of this propellant combination. The engine was very clean after repeated runs using the mixed mercaptans and the above-described acid.

A number of performance tests were made in a 220 pound thrust rocket engine to determine the elfect of varying the acid mercaptan ratio in the combustion chamber of the engine. Using the mercaptan mixture described in Example 9 and white fuming nitric acid, a ratio of acid to mercaptans of 4.2:1 would be stoichiometric for complete oxidation of the mercaptans. It was found that the specific impulse, i. e., the number of pounds thrust per pound of fuel per second, went through a maximum as the ratio of acid to mercaptan was varied and that this maximum existed when the acid content of the acid mercaptan mixture was from about 80% to 95% of stoichiometric. Further tests with pure mercaptans showed that similar maxima existed for mixtures of these materials with fuming nitric acid and with fuming nitric acid containing the ignition promoters described herein. Accordingly, it is preferred to operate thrust engines using acid and mercaptan injection rates such that the acid mercaptan ratio is 5% to 20% below stoichiometric.

The foregoing examples have been limited to illustrations of Self-igniting mixtures of the mercaptans alone with fuming nitric acid. It is not necessary, however, that pure mercaptans be used. In practical operation it is possible and often desirable to blend the mercaptans with up to 50% by volume of hydrocarbon fuel.

EXAMPLE 17 Individual mercaptans and the mixed mercaptans of Example 9 were diluted with a straight run petroleum distillate and red fuming nitric acid was added to the resultant mixture at room temperature. The maximum amounts of distillate which could be added to the mercaptans without losing the property of self-ignition upon addition of nitric acid were determined. The distillate employed in the tests tabulated below had a 10% point of 215 F. and a 90% point of 250 F. on an ASTM D-86 distillation and contained 43% paraffins, 48% naphthenes, and 9% aromatics. Approximately equivalent results were obtained with cracked distillates and with wider boiling cuts such as gasoline and kerosene.

Maximum Dilution with Petroleum Distillate for Self-Ignition with Farming Nitric Acid Mercaptan Vol. percent Isopropyl mercaptan 30 n-propyl mercaptan 30 i-butyl mer p 30 n-butyl mercaptan Mixed mercaptans Allyl mercaptan EXAMPLE 18 Although it has been found that pure tertiary mercaptans are not self-igniting in HNO3, considerable concentration of the C1 or C5 tertiary compounds may be present in the primary orsecondary mercaptans of this invention without seriously restricting the utility of these materials as propellents. The exact quantity of tertiary mercaptan that can be tolerated will depend upon the intended use, but it may be as high as 30 to 50%. In some cases it may be more practical tornanufacture such mixtures than it is to make the pure primary and secondary mercaptans, and mixtures of primary and/or secondary mercaptans containing substantial quantities of tertiary mercaptans are, therefore, within the scope of this invention.

The self-igniting character of mixtures of tertiary mercaptans with non-tertiary mercaptans is evidenced by the following tabulated results obtained pursuant to the method of Example 1, using a mixture of sulfuric and white fuming nitric acids as the oxidizing agent.

V01. percent Vol. percent Ignition Sec.-butyl t-butyl 29 2 2g??? Mercaptan Mercaptan seconds The mixtures of primary and secondary mercaptans with the tertiary mercaptans should contain at least 50% by volume of the primary and secondary mercaptans and preferably should predominate in these materials if overall satisfactory properties, including freezing point, are to be insured.

Obviously, many modifications and variations of the in vention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. The method of starting a fire by autoignition which comprises mixing at least one mercaptan of the group consisting of primary and secondary acyclic mercaptans containing from 1 to 5 carbon atoms per molecule with an oxidizing agent consisting predominantly of fuming nitric acid.

2. A fuel, characterized in that it is self-igniting upon addition of concentrated nitric acid, consisting essentially of a mixture of acyclic mercaptans containing 1 to 5 carbon atoms per molecule, said mixture containing at least 50% by volume of mercaptans selected from the group consisting of primary mercaptans and secondary mercaptans.

3. The method of operating a thrust engine which comprises introducing a mixture of acyclic mercaptans containing from 1 to 5 carbon atoms per molecule, said mixture containing at least 50% by volume of mercaptans selected from the group consisting of primary mercaptans and secondary mercaptans, and fuming nitric acid into the combustion chamber of the engine to form an ignited propulsion charge and ejecting the burning charge from the combustion chamber at high velocity.

4. The method as defined in claim 3, wherein the quantity of fuming nitric acid introduced into the combustion chamber is from 80 to of the amount stoichiometrically required to oxidize the mercaptans.

5. The method as defined in claim 1, wherein the nitric acid contains a minor amount in the range from about 2 to 20% by weight of sulfuric acid.

6. The method as defined in claim 1, wherein the nitric acid contains a minor amount in the range from about 2 to 20% by weight of nitrosyl sulfuric acid.

7. The method as defined in claim 1, wherein the nitric 7 acid contains a minor amount in the range from about 2,033,877 U 2% to 20% by weight of nitrogen dioxide. 2,328,709 2,489,051 References Cited in the file of this patent 2,557,018 UNITED STATES PATENTS 5 2,573,471

1,742,263 Koch Jan. 7, 1930 8 Burk Mar. 10, 1936- Crandall et a1. Sept. 7, 1943 Sayward et a1. Nov. 22, 1949 Viles June 12, 1951 Malina et a1. Oct. 30, 1951 

3. THE METHOD OF OPERATING A THRUST ENGINE WHICH COMPRISES INTRODUCING A MIXTURE OF ACYCLIC MERCAPTANS CONTAINING FROM 1 TO 5 CARBON ATOMS PER MOLECULE, SAID MIXTURE CONTAINING AT LEAST 50% BY VOLUME OF MERCAPTANS SELECTED FROM THE GROUP CONSISTING OF PRIMARY MERCAPTANS AND SECONDARY MERCAPTANS, AND FUMING NITRIC ACID INTO THE COMBUSTION CHAMBER OF THE ENGINE TO FORM AN IGNITED PROPULSION CHARGE AND EJECTING THE BURNING CHARGE FROM THE COMBUSTION CHAMBER AT HIGH VELOCITY. 